How do I enter buildings into my map?

Im having trouble figuring out how to make or color buildings that i can enter, but i have made cube but i cant figure out how to color them. and how do i enter weapons and characters? im new to all of this

You need to create a material for your project. Under your project pane, click on Create -> Material.

You can then choose a shader, and then a texture for your material, or simply select the color and alpha values. Once your material is created in the project pane, simply drag it onto your cube or whichever object you wish to color.

For more information on Unity's support for materials and shaders go here...